3.1 What is light and how is it absorbed and measured?

Figure 5. Light penetration in the skin
(attenuation down to 1% occurs for light wavelengths of 250-280 nm
at around 40 μm depth; for 300 nm at 100 μm; for 360 nm at 190 μm;
for 400 nm at 250 μm; for 700 nm at 400 μm; for 1.2 μm at 800 μm;
for 2 μm at 400 μm; for 2.5 μm at 1μ; and for 400 μm at 30 μm)

Light is electromagnetic radiation which is visible by the
human eye and has a wavelength between 400 and 780 nm. (1 nm =
10-9 m). Visible
light constitutes a very small part of the whole
electromagnetic
spectrum and for instance,
ultraviolet radiation
covers the range from 100 nm up to 400 nm, and
infrared (IR)
radiation from 780 nm up to 1 mm. The UV and IR ranges are also
subdivided into narrower bands (UVA/UVB/UVC and IRA/IRB/IRC).
The Sun emits radiation over the whole electromagnetic spectrum
but the Earth’s atmosphere blocks UVC and some UVB.

The upper layers of the skin absorb most of the UV, IR and
visible light they
receive. Visible and IRA radiation penetrate deepest, down to
the dermis. In the eye, UVC, IRB and IRC are absorbed by the
cornea so they go
no further. UVA and UVB go as far as the
lens. Visible and IRA
reach the retina, and more
so in children than in adults.

The temperature of the skin or the eye increase when they
absorb radiation, particularly IR. UV can also cause chemical
reactions in the body, some of which are beneficial, and some
which are harmful. Radiation of a specific wavelength is
absorbed by parts of molecules in the body called chromophores
and this produces a photochemical
reaction. UV is the most photochemically active type
of radiation and is absorbed by many molecules in the skin and
in the eye.

Exposure to light is measured as the energy of the radiation
that is received per unit area. Exposure calculations have to
consider the detailed wavelength spectrum of the incident
radiation, the medium it goes through, the chemical reaction
involved and how well the chromophore absorbs light of each
wavelength.

Skin exposures also depend on the distance from the light
source. European standards use two different types of
measurements depending on the potential use of the light
source:

Lamps designed to illuminate a large area such as
workplaces or shopping areas, are usually placed in the ceiling
relatively far away from users so these lights are evaluated at
a distance that produces a certain level of illumination (500
lx)

People who use task lights and downlights are more likely
to look directly into the light source so these lights are
tested at a distance of 20 cm.

3.2 How can light affect biological systems?

Excessive amounts of light or heat can be harmful, and the
body has methods of protection against it. For instance, very
bright sources make people close the eyes and turn their face
away so they are not focused on the bright light for any
substantial length of time. The iris responds to bright light by
constriction so it can regulate the amount of light that enters
the eye. Pain and reflexes also make people move away from
sources of excessive heat so they protect the skin. However,
these natural aversion methods are not always sufficient to
avoid damage.

The heat absorbed from light sources can be enough to damage
cells permanently.
Superficial damage can be repaired by new cells deep into the
skin, and this process is used in some cosmetic treatments.
However, deeper burns need hospital treatment and sometimes skin
grafts. The eye is rarely harmed by excessive heat from domestic
lamps but light from pulsed lamps and lasers can very quickly
cause burns.

UV light can damage tissues indirectly by producing very
reactive compounds (mainly free
radicals and reactive oxygen) that go on to damage
cells. The
retina is very
susceptible to this type of damage and is particularly
vulnerable to radiation of short wavelength. The skin has agents
that remove these very reactive species or repair damaged cells,
and these are effective for low exposures but higher exposures
can lead to cell death. The eye contains pigments that combine
with reactive oxygen species and protect the retina. There are
more of those pigments in the eyes of children than in older
people so, with age, the retina can be more sensitive to damage,
which leads to age-related macular degeneration. On the other
hand, the lens becomes
yellower with age and absorbs some blue light so the retinas of
older people have some natural protection.

UV exposures that are not high enough to cause immediate burns
can lead to an accumulation of damage and to loss of
collagen and skin
aging, as well as to skin
cancer. Prolonged exposure of the eye to UV can make the
edge of the lens cloudy and
also lead to melanomas and tumors. UV and IR lights can induce
cataracts.